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8 3/4 Rear End Case I.D and Gear Questions #1183780 02/22/12 12:07 AM 02/22/12 12:07 AM
Joined: Jun 2010 Posts: 287 Iowa, US oldsklharo OP enthusiast
oldsklharo OP enthusiast Joined: Jun 2010 Posts: 287 Iowa, US	I have a '66 8 3/4 sure grip rear end with a really low gear. I haven't figured out the ratio yet but it is screaming at 65 mph. I want to go with a taller gear to bring my highway cruising RPM down but don't know what case I have. How do I determin what I have? I have a gnarly built 383 BB and I'm thinking about a 3:23....... Am I doing the right thing?

Re: 8 3/4 Rear End Case I.D and Gear Questions [Re: oldsklharo] #1183781 02/22/12 01:31 AM 02/22/12 01:31 AM
Joined: Jan 2003 Posts: 75,005 U.S.S.A. JohnRR I Win
JohnRR I Win Joined: Jan 2003 Posts: 75,005 U.S.S.A.	look at it ??? it will have a part number on the drivers side of the case up near the pinion yoke , the last 3 digits are what you wantto know , it will either be a 741 or a 742 if original , but wit ha gear change do not bank on that , or a 489 . A 3.23 would not be a bad choice for a highway cruiser.

Re: 8 3/4 Rear End Case I.D and Gear Questions [Re: JohnRR] #1183782 02/22/12 11:57 AM 02/22/12 11:57 AM
Joined: Jun 2010 Posts: 287 Iowa, US oldsklharo OP enthusiast
oldsklharo OP enthusiast Joined: Jun 2010 Posts: 287 Iowa, US	Thanks for the info I'm 37 and not real versed on identifying Mopar parts so I apologize if some of my questions seem foolish. I am a die hard Mopar fan but as you all know old school Mopar cars/trucks are just a different breed Anyone know what the RMP difference is between gear sets? Is there a chart somewhere? I mean, is there 100-150 RMP difference between many of the different gears?

Re: 8 3/4 Rear End Case I.D and Gear Questions [Re: oldsklharo] #1183783 02/22/12 12:27 PM 02/22/12 12:27 PM
Joined: Nov 2003 Posts: 3,428 Dandridge TN D Dabee master
Dabee master D Joined: Nov 2003 Posts: 3,428 Dandridge TN	Quote: Anyone know what the RMP difference is between gear sets? Is there a chart somewhere? I mean, is there 100-150 RMP difference between many of the different gears? Go to this site and plug in the numbers. http://www.wallaceracing.com/Calculators.htm

Re: 8 3/4 Rear End Case I.D and Gear Questions [Re: Dabee] #1183784 02/22/12 12:34 PM 02/22/12 12:34 PM
Joined: Jun 2010 Posts: 287 Iowa, US oldsklharo OP enthusiast
oldsklharo OP enthusiast Joined: Jun 2010 Posts: 287 Iowa, US	Quote: Quote: Anyone know what the RMP difference is between gear sets? Is there a chart somewhere? I mean, is there 100-150 RMP difference between many of the different gears? Go to this site and plug in the numbers. http://www.wallaceracing.com/Calculators.htm Hmmm, FYI......I tried that link but my security on my computer blocked it. The alert said it was infected with a "TrojanHorse" virus I'll try plugging it into my browser

Re: 8 3/4 Rear End Case I.D and Gear Questions [Re: oldsklharo] #1183785 02/22/12 12:37 PM 02/22/12 12:37 PM
Joined: Jun 2010 Posts: 287 Iowa, US oldsklharo OP enthusiast
oldsklharo OP enthusiast Joined: Jun 2010 Posts: 287 Iowa, US	Crap! My computer lets me go to the Wallace Racing site but won't let me click on the Automotive Calculator, UUGH!

Re: 8 3/4 Rear End Case I.D and Gear Questions [Re: oldsklharo] #1183786 02/22/12 12:43 PM 02/22/12 12:43 PM
Joined: Jun 2010 Posts: 287 Iowa, US oldsklharo OP enthusiast
oldsklharo OP enthusiast Joined: Jun 2010 Posts: 287 Iowa, US	I found another one that works...... http://www.rearendparts.com/pages.php?pageid=16

Re: 8 3/4 Rear End Case I.D and Gear Questions [Re: oldsklharo] #1183787 02/22/12 01:45 PM 02/22/12 01:45 PM
Joined: Jan 2003 Posts: 32,930 Grand Prairie,Texas stumpy I Win
stumpy I Win Joined: Jan 2003 Posts: 32,930 Grand Prairie,Texas	From our TECH ARCHIVES. http://vexer.com/automotive-tools/speed-rpm-calculator

Re: 8 3/4 Rear End Case I.D and Gear Questions [Re: stumpy] #1183788 02/22/12 01:50 PM 02/22/12 01:50 PM
Joined: Jun 2010 Posts: 287 Iowa, US oldsklharo OP enthusiast
oldsklharo OP enthusiast Joined: Jun 2010 Posts: 287 Iowa, US	Quote: From our TECH ARCHIVES. http://vexer.com/automotive-tools/speed-rpm-calculator Now that is bad ass!! Thanks!

Re: 8 3/4 Rear End Case I.D and Gear Questions [Re: oldsklharo] #1183789 02/22/12 06:32 PM 02/22/12 06:32 PM
Joined: Jan 2003 Posts: 25,791 Rio Linda, CA John_Kunkel Too Many Posts
John_Kunkel Too Many Posts Joined: Jan 2003 Posts: 25,791 Rio Linda, CA	Forget the calculators and determine the actual ratio by raising the rear and turning a tire...turn the tire exactly one turn and count the driveshaft turns. The INTERNET, the MISinformation superhighway

Re: 8 3/4 Rear End Case I.D and Gear Questions [Re: John_Kunkel] #1183790 02/22/12 07:00 PM 02/22/12 07:00 PM
Joined: Jun 2010 Posts: 287 Iowa, US oldsklharo OP enthusiast
oldsklharo OP enthusiast Joined: Jun 2010 Posts: 287 Iowa, US	Quote: Forget the calculators and determine the actual ratio by raising the rear and turning a tire...turn the tire exactly one turn and count the driveshaft turns. Yes I am going to do exactly that but in the mean time I want to find a gear that will get me a good highway cruising RPM. Looks like a 3:23 will get me to about 2500 and a 3:55 will get me to around 2800 both at 65mph with the wheel/tire package I am running. I'm sure I have a 3:91 for a 4:10 in it now, just need to verify for sure which one.........either way it's coming out

Re: 8 3/4 Rear End Case I.D and Gear Questions [Re: oldsklharo] #1183791 02/22/12 10:07 PM 02/22/12 10:07 PM
Joined: Jun 2010 Posts: 287 Iowa, US oldsklharo OP enthusiast
oldsklharo OP enthusiast Joined: Jun 2010 Posts: 287 Iowa, US	Well I have a 3:91 gear and a 489 case. Guess Ill go search for a 3:23/489

Re: 8 3/4 Rear End Case I.D and Gear Questions [Re: oldsklharo] #1183792 02/22/12 11:32 PM 02/22/12 11:32 PM
Joined: Aug 2003 Posts: 43,172 Bend,OR USA C Cab_Burge I Win
Cab_Burge I Win C Joined: Aug 2003 Posts: 43,172 Bend,OR USA	Quote: Well I have a 3:91 gear and a 489 case. Guess Ill go search for a 3:23/489 3.23 in a 741 or 742 case is way better You can swap the pinion yokes with no troubles on them, not so on the 489 cases Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)

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